At one farm in Northern Ireland, cows are giving up green grass in favor of green power. In order to decrease his reliance on fossil fuels for electricity, farmer William Taylor created the Livestock Power Mill, a treadmill that generates power as cows walk on it. It may seem like a kooky idea, but Taylor could really be onto something: According to his calculations, if the world’s 1.3 billion cattle used treadmills for eight hours a day, they could produce six percent of the world’s power.

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Cows are locked into a pen on top of a non-powered, inclined belt. To avoid sliding down the incline, the cow needs to walk, which turns the belt. As the belt turns, it spins a gearbox, which powers a generator. A feed box hooked to the front of the device keeps cows occupied and happy. One cow can produce about two kilowatts of electricity, enough energy to power four milking machines.

It may seem cruel to make cows sweat it out on a treadmill, but the routine is actually quite similar to the animals’ normal behavior. Cows walk about eight hours a day while grazing. Doing that walking on a treadmill provides the same amount of exercise with the added bonus of renewable power production.

For now, the Livestock Power Mill is just a prototype on Taylor’s farm. But if the idea catches on, we could see farms all over the world employing the useful device. Taylor estimates that a small farm could earn back a 50-cow system’s $100,000 price tag in a mere three years.

Via Popular Science